This invention relates to a supporting structure for an underwater structure, and particularly to a supporting structure suitable for supporting the weight and buoyancy of an underwater structure such as an axial-flow bulb-type turbine disposed in a waterway.
As an example of an underwater structure disposed in parallel to a water flow, there is an axial-flow bulb-type turbine which comprises a bulb body containing therein a generator, etc., and guide vanes and runners each mounted on the bulb body. The axial-flow bulb-type turbine is disposed in water flowing in a waterway surrounded by a concrete construction and is supported by stay vanes and supporting structures such as a bulb body stay, and vibration prevention stays. The stay vanes each extend radially from the bulb body to the concrete construction of the waterway, and one end of each stay vane is fixed to the bulb body and the other end to the concrete construction. The supporting structures are fixed to the bulb body and the concrete construction to support the bulb body at places axially distant from the stay vanes.
The supporting structure generally is classified as a stay type or a pedestal type.
An example of the axial-flow bulb-type turbine employing a stay type supporting structure is disclosed in "Hydroelectric Power Generation" (La Houille Blanche/N 5/6-1982). These type of water turbines have been used in Idaho-Falls Hydropower Plant, Pelton Hydropower Plant, etc. An example of the axial-flow bulb-type turbine employing a pedestal type supporting structure is used in Rock Island Hydropower Plant, which is disclosed in Water Power & Dam Construction 7-1978".
These two types of the supporting structure each have merits and demerits.
In case of the stay type supporting structure, the supporting structure, such as the bulb body stay or the vibro-isolating stay, is constructed so as to restrict an axial deformation of the bulb body due to its thermal expansion in cooperation with the stay vanes, so that the axial deformation induces bending stresses in the supporting structure. As a result, the supporting structure is subjected to axial stresses due to the weight and the buoyancy as well as the bending stresses, whereby the stresses induced in the supporting structure are raised.
The bending stresses increase as the flexural rigidity of the supporting structure increases. On the other hand, when the flexural rigidity is decreased, it is difficult for the supporting structure to have a real cross-section area enough to support the bulb body.
In case of the pedestal type supporting structure, the supporting structure has a sliding face which is slidable in a direction of deformation of the bulb body due to thermal expansion. Therefore, bending stresses induced in the supporting structure by the thermal expansion of the bulb body are much less than in the stay type supporting structure. However, in the pedestal type supporting structure, it is necessary to seal airtightly the sliding face to prevent corrosion and to conduct periodical inspection and maintenance.